S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Mechanical Aerospace Engineering (기계항공공학부) Theses (Ph.D. / Sc.D._기계항공공학부)
Study of Anode Supported GDC/YSZ Bi-layered Electrolyte Solid Oxide Fuel Cell via Cold Press Process
콜드 프레스 공정을 이용한 음극지지체 GDC/YSZ 전해질 이중층 고체산화물 연료전지 연구
- 공과대학 기계항공공학부
- Issue Date
- 서울대학교 대학원
- solid oxide fuel cell (SOFC) ; intermediate temperature ; gadolinia doped ceria (GDC) ; bi-layered electrolyte
- 학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2014. 2. 차석원.
- The fact that hydrogen will be the last energy source became no more attractive to us. The main issue is which kind of energy conversion device will be going to survive in the future. Since many researchers highlighted the fuel cell as the next generation power source, a lot of researched have been conducted to commercialize it.
SOFCs have many advantages in comparison with typical PEMFCs which have shown water management problem, usage of novel catalyst, patent issue for polymer electrolyte, expansive graphite bipolar plate and CO poisoning. So many researchers in energy field have been thought SOFC would be the promising device.
But the main bottle neck for the commercialization of SOFC has been its high operation temperature. It can cause thermal mismatch between MEA, nickel agglomeration, reactions between component materials, restricted sealant choice and expensive interconnecter material. So we focused our interest to IT-SOFC. Its temperature position can avoid many problems of HTSOFC and LT-SOFC maintaining the competitiveness of original SOFCs characteristics. Moreover, process cost issue about compaction, sintering and more complicated high temperature process is one of the bottle-neck for commercialization.
In this study, we studied the lower the operating temperature in order to solve these problems, while reducing the thickness of the electrolyte and dropping the number of steps compared to the conventional method.
First, we prepared the bi-layer electrolyte solid oxide fuel cell which was deposited yttria-stabilized zirconia (YSZ) and gadolinia dopted ceria (GDC) which is high ionic conductivity material at low and intermediate temperature. This structure is verified that high performance, sufficient durability and operation in a low temperature.
It is confirmed the successful deposition of YSZ through the scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). We obtained a 50% improvement power density and 5% higher open circuit voltage (OCV) than the output of pure GDC electrolyte cell at 600 degrees Celsius. We confirmed that the YSZ layer prevent enhancing electronic conductivity and micro crack which can make voltage-drop and enlarge ohmic loss in order to reduce GDC layer by SEM image and EIS.
Second, because we verified effect of YSZ/GDC bi-layer electrolyte structure to the performance, it is studied about bi-layer electrolyte anode supported type solid oxide fuel cell structure which could have higher power density. Various wet ceramic processes and thin film deposition processes are employed to electrolyte deposition method to calcined anode substrate. Finally, we established cold press process that can sinter from substrate to electrolyte at one step process.
In order to enhance the controllability and uniformity of thickness of the layer, spray dry coating method that can control about micron range using powder vehicle is employed. The compressed dry powder substrate-bi-layer electrolyte was co-sintered, and we fabricated LSCF-GDC cathode using screen printing method. It is possible as compared with the wet method, to reduce the sintering step one or more times, forming method such a great advantage in time and cost.
We obtained power density of 210mW/cm2 at 600 degrees Celsius, and 409mW/cm2 at 800 degrees Celsius from prepared bi-layer electrolyte cell. It is 0.1V higher open circuit voltage and 15% higher maximum power density rather than one of non bi-layer electrolyte cell.
Third, we have improved the fabrication process to increase the performance of bi-layer electrolyte cell via cold press process. In order to make porous anode substrate, the PMMA powder was mixed with anode substrate powder which is used previous bi-layer electrolyte cell fabrication. And it is achieved the power density is 460mW/cm2 at 700 degrees Celsius. Further, by adding one functional layer between the anode substrate and electrolyte layer, to complement the reduction in mechanical strength that can occur in the case of porous anode support, to improve the electrochemical performance by cold press process. As a result, it was possible to obtain a power density and mechanical strength is enhanced, and is more stably maintained even during operation.
Finally, we modeled and calculated the compaction behavior during cold press process using COMSOL multiphysics. The Surface von Mise stress was predicted using the Drucker-Prager Cap model. Through this model, we were able to determine at the time of production to minimize the damage, and the condition of the powder. In addition, through the modeling, the basic direction for the production of large-scale cells could be present.